Heating, ventilation, and air-conditioning (HVAC) systems of the vehicle passenger compartment are known in which the internal heat exchanger (evaporator) and the heat exchanger for heating (condenser) are placed inside the internal air flow, with the heat exchanger for heating placed in the part of the flow located behind the internal heat exchanger.
The air stream of the internal air flow, which originates from the environment or the passenger compartment, or is obtained by mixing the two sources, is first cooled in the internal heat exchanger to dew point temperature in order to remove moisture from it. The divider of the air flow divides the air stream of the internal air flow into two air streams, of which one is heated on the heat exchanger for heating, while the other bypasses the heat exchanger for heating, and then the two air streams, passing through the flow collector, is introduced into the vehicle passenger compartment (cabin) as single stream: the air stream entering the cabin is a mixture of both air streams, the stream of heated air, as well as the air stream that bypasses the heat exchanger for heating. Therefore, the temperature of air entering the vehicle cabin can be controlled by altering the relative amount of heated air in the total air flow.
When the heating mode is selected as the working mode of the refrigerant cycle, the internal heat exchanger can also serve as a heat radiator, provided no air drying is required, i.e. when the temperature of the air coming into the internal air flow is below the dew point temperature. This implies that the air of the internal air flow is first pre-heated in the internal heat exchanger and then heated in the heat exchanger for heating.
When an HVAC system with multiple zones is desired, the internal air flow is, after the internal heat exchanger, divided into partial air flows, one for each required zone. Each of the partial air flows has its own air flow divider, which allows for independent control of the temperature of the air mix at the outlet of each partial air flow.
The stream of the external air flow, which always originates from the environment, exchanges heat in the external heat exchanger before returning to the environment. The external heat exchanger is integrated with the internal heat exchanger and the heat exchanger for heating, which allows for heat exchange between internal and external air flows.
The selection of the refrigerant cycle working mode is achieved by altering the topology of the refrigerant flow, by a suitable selection of switching valves positions. The choice of cooling mode as the working mode of the refrigerant cycle leads to discharge of the refrigerant from the compressor discharge outlet, and its' cycling through the heat exchanger for heating via the external heat exchanger, adjustable throttle valve (electronically controlled throttle valve), internal heat exchanger and accumulator back to the compressor. The choice of the heating mode as the working mode of the refrigerant cycle leads to discharge of the refrigerant from the compressor discharge outlet, and its' cycling through the heat exchanger for heating via the adjustable throttle valve (the second electronically controlled throttle valve), external heat exchanger and accumulator back to the compressor.
An HVAC system can additionally be improved by collecting the excess heat from other devices in the vehicle, e.g. waste heat from the engine or electronic devices. The HVAC refrigerant cycle is then thermally connected with the cooling system of the device by introducing an internal heat exchanger for collecting the waste heat. The additional branch of the refrigerant flow is introduced into the refrigerant cycle from the internal heat exchanger via the second adjustable throttle valve, internal heat exchanger for collecting the waste heat to the accumulator. The presence of the additional branch of the refrigerant flow is an advantage in both working modes of the refrigerant cycle, because it allows for collecting additional heat into the HVAC system when needed, but also allows for using the HVAC system to facilitate cooling of the devices in the vehicle.
The refrigerant cycle is described in the USA patent specification U.S. Pat. No. 7,121,103 B2 2006.10.17 VEHICLE AIR CONDITIONING SYSTEM of the Japanese company Denso Corporation discloses an example of a HVAC system in which the selection of the working mode (heating, cooling or dehumidifying) is achieved by altering the topology of the refrigerant flow, which is achieved by using several switching and adjustable throttle valves.
Such structure limits the efficiency due to unavoidable losses during the refrigerant flow through an open switching valve, and the large number of active components reduces the reliability of the system. Besides, this system cannot be used for air dehumidification when external temperature is below 0° C., since the evaporator is at the same pressure as the external heat exchanger, and such use would lead to its freezing. The known solutions to this problem further add to the complexity of the system by adding a larger number of switching and adjustable throttle valves, so as to allow for division of working pressures of the evaporator and the external heat exchanger. An additional problem is also the change in the working mode from cooling to heating, due to possible fogging of the windshield if the water from the internal air flow condenses on the internal heat exchanger in the cooling mode.
The efficiency is especially limited when fresh air from the environment is the source of the internal air stream, since in that case it is necessary to release the already conditioned air from the passenger compartment into the environment. Known solutions to this problem include a recycling heat exchanger integrated into the refrigerant cycle that enables recovery of the heat from the air stream that leaves the vehicle passenger compartment. Unfortunately, great complexity of such solutions does not justify the small gains in efficiency, so they are not applied in practice. Additionally, such solutions are applicable only in the heating mode, whereas in the cooling mode the gained increase in efficiency would be nullified by losses on the additional elements of the refrigerant cycle needed for the realisation of these solutions.
In a disclosed patent specification U.S. 2013/0042637 A1 2013.02.21 MODULAR AIR CONDITIONING SYSTEM WITH HEAT PUMP FUNCTIONALITY by Visteon Global Technologies Inc. an HVAC system without alteration of refrigerant flow topology is described. The air streams of the internal and external air flows are obtained by mixing the warm air stream, heated on the heat exchanger for heating, and a cool air stream cooled on the heat exchanger for cooling. Temperatures and mass flows of air in the internal and external air stream can independently be adjusted by changing the ratio of the mass flows of warm and cool air stream, which indirectly also changes the working mode.
Although this system has a very simple refrigerant cycle, a great number of regulation flaps for control is needed, so that the overall complexity of the system remains high. The complexity is also negatively affected by the required use of advanced techniques of automatic control, as well as a significant number of sensors.
A particular disadvantage of this system is apparent when it is requested to work in the dehumidifying mode, which is a necessary working mode whenever the quality of air in the environment is unsatisfactory. In that case there is no air stream that brings the air from the environment to the heat exchanger, so that it is not possible to use the environment as a heat source or sink. It is therefore impossible to cool the air in the passenger compartment, whereas air heating is possible only with an additional source of heat, thereby significantly reducing the efficiency of the system.
An additional disadvantage of this system is that it does not allow for changing the working mode from the cooling or dehumidifying mode into the heating mode when the external temperature is below 0° C. The reason is that in the cooling and dehumidifying modes the condensed water collects at the heat exchanger for cooling. The change of the working mode into the heating mode would bring the cold external air into contact with condensed water, which may lead to formation of ice on the heat exchanger for cooling, causing thus a breakdown in system function, or its permanent damage.